1. Field of the Invention
The present invention relates to thin film metallization and laser beam stimulated chemical processing and, more particularly, to thin film metallization for the interconnection of circuitry and laser beam stimulated chemical processing for microelectronic circuit fabrication.
2. Description of Related Art
There exists a need for a capability for making selective connections between the individual conductors on individual microelectronic circuit chips and on multi-chip modules. Laser beam stimulated processes can be employed to form these connections. An attractive candidate material for forming such electrically conductive links is tungsten metal because there are well established laser CVD processes by which it can be deposited. Details regarding several of these processes are set forth in A. W. Johnson and K. E. Greenberg, in "Laser-Chemical Deposition and Etching on the Metallization Level of Integrated Circuits", edited by V. Donnelly, I. Herman, and M. Hirose, Mater. Res. Soc. Proc. 75, pittsburgh, PA, pp. 645-649 (1987) and J. G. Black, S. P. Doran, M. Rothschild, J. H. C. Sedlacek, and D. J. Ehrlich, in "GaAs Circuit Restructuring by Multilevel Laser-Direct Written Tungsten Process", edited by V. Donnelly, I. Herman, and M. Hirose, Mater. Res. Soc. Proc. 75, Pittsburgh, PA, pp. 651-655 (1987), both of which articles are hereby incorporated by reference.
It is important to note that these two articles describe research in which gold, aluminum and/or polycrystalline silicon were intra-chip metals to be contacted. For inter-chip interconnects, on the other hand, most persons skilled in the art prefer copper because it has a relatively high conductivity (compared to, e.g., aluminum and poly-silicon) and because it is relatively inexpensive (compared to, e.g., gold).
The tungsten laser CVD processes described in the two articles cited above are incapable of being reliably used to form an electrical interconnection of copper conductors. In an interconnect application, to which the method of the present invention is aimed, if copper and tungsten were to be employed, it would be important that the contact between the tungsten and copper be low-resistance, that is, that it be metallurgically "good." Yet, it is well known that tungsten is insoluble in copper. Those persons skilled in the art know that copper-tungsten "alloys" are in actuality simply mixtures of the two metals prepared by allowing molten copper to seep into the pores of a tungsten material. This fact has impeded attempts to form good electrical interconnection between copper and tungsten at low temperatures. For example, Takashi Nagasaka and a number of colleagues were, as reported in T. Nagasaka, Y. Ootani, K. Ban, S. Konda, and T. Sonobe, "The Connection of a Copper Conductor with a Tungsten Conductor on an Alumina Multilayer Substrate", Proceedings of IMC, pp. 255-261 (1986), only able to achieve satisfactory contact between copper and tungsten after plating the copper on the tungsten and then firing the composite at 900.degree. C. From the foregoing it is clear that a low temperature (i.e., &lt;500.degree. C.) process that forms satisfactory low-resistance, well-adhering contact between laser-deposited tungsten metal and prepatterned copper conductors is still needed. The requirement that the process be carried out at low temperatures is especially important since medium-film interconnect substrates can employ dielectrics with low decomposition temperatures, for instance polyimide.
For a full understanding of the prior art related to the present invention, certain attributes of nickel must also be discussed. Nickel is known to form a number of alloys with tungsten, which alloys are stable at low temperatures. J. S. Lee and several of his colleagues have investigated compacts formed by W-Cu, W-Ni, and W-Cu-Ni. The results of their investigation are reported in J. S. Lee, W. A. Kaysser, and G. Petzow, "Microstructural Changes in Tungsten-Copper and Tungsten-Copper-Nickel Compacts During Heating Up for Liquid Phase Sintering", Mod. Dev. Powder Metall., v. 15, pp. 489-506 (1985). Lee et al. showed that at temperatures between 700.degree. C. and 900.degree. C. W-Cu-Ni compacts behave qualitatively similar to W-Ni compacts. This may be contrasted with the behavior of W-Cu compacts alone, for which no wetting is observed below the melting point of copper, that is, 1080.degree. C. In other words, the addition of nickel improves the wetability of copper on tungsten surfaces. Above 900.degree. C., but below the melting point of copper, copper takes nickel into solid solution in W-Cu-Ni compacts, reducing the W-Ni interaction (wetting). In an extension of this work, reported in J. S. Lee and I. H. Moon, "Effect of Tungsten Particle Size on Infiltration Process and Microstructure in Nickel-Doped Tungsten-Copper Electric Contacts", Horiz. Powder Metall. Proc. Int. Powder Metall. Conf., Exhib. pp. 1115-18 (1986), Lee and another colleague reported that the addition of nickel also affects the resulting tungsten network structure.
From the bulk CVD process literature (not a laser-enhanced process, as per the teachings of the present invention), the work of Kamijima et al. warrants mention as "related" to the present invention. Kamijima et al.,s research is reported in A. Kamijima, S. Ito, H. Momotani and N. Yoneda, "Tungsten Coatings on Iron, Nickel, Copper and Stainless Steel Obtained by CVD Method", Journal of the Metal Finishing Society of Japan, V. 39, pp. 315-322 (1988) (in Japanese). Kamijima et al. have performed experiments in which tungsten was deposited from WC1.sub.6 onto iron, nickel, copper, and stainless steel substrates. Despite their claims that tungsten adheres well on both nickel and copper, it is clear from SEM photographs of surface morphology in Kamijima et al.'s article that the tungsten covers the surface of the nickel in a much smoother, less porous manner. While porosity may not adversely affect adhesion, it might definitely affect electrical resistance--a less porous deposit is clearly more desirable. Kamijima et al. do not mention the electrical implications of porosity.
From the foregoing, several things may be understood by those persons skilled in the art. First, it is clear that copper-to-tungsten contacts are not as easy to form, and are probably not as reliable as nickel-to-copper or nickel-to-tungsten contacts, even if formed at high temperatures. Second, while there are hints in the literature as to the scientific basis underlying applicant's methodology, the present invention is neither disclosed nor suggested by the prior art teachings. The shortcomings Of the techniques suggested by the prior art are only likely to be aggravated at the temperatures of interest for laser CVD processing, that is, in the range of 300.degree.-500.degree. C.